In the pneumatic tire having the center rib mentioned above, since a rigidity of the center portion of the tread surface is enhanced, there is a tendency that a travel motion becomes stable under such a condition that a severity (a travel severity) is low, and a steering stability at a time of changing lane from a rectilinear travel (hereinafter, refer to as “lane change stability”) is improved. On the other hand, if the center rib is made wide so as to make the rigidity too high, a grounding property with respect to a road surface is deteriorated under such a condition that the severity is high, for example, at a marginal cornering time, a braking time and the like, whereby a sufficient grip cannot be obtained. Accordingly, the steering stability at a time of cornering (hereinafter, refer to as “cornering stability”) and a braking performance are deteriorated.
In brief, there is a tendency that if the center rib is made narrow, the lane change stability is lowered, and if the center rib is made wide, the cornering stability and the braking performance are lowered, and both are in an incompatible relationship with regard to the width of the center rib. In general, in a tire having a wide grounding width or a tire having a low flattening, since the center rib tends to become wide from a relative relationship to the other region of the tread surface, there is fear that the cornering stability and the braking performance are deteriorated. On the contrary, there can be considered such a countermeasure for making the center rib low in its rigidity by segmenting with a narrow groove or a sipe or using a soft rubber compounding. However, in that case, the lane change stability is deteriorated.
In a pneumatic tire described in the following Patent document 1, a tread band is separated into two sections in a tire width direction, an elastic modulus of a rubber outside a vehicle is set to 12 to 16 MPa, and an elastic modulus of a rubber inside the vehicle is set to 7 to 11 Mpa. However, this is for the purpose of enhancing a steering stability at a time of cornering on a dry road surface by the rubber outside the vehicle, and enhancing a ride comfort and a steering stability on a wet road surface by the rubber inside the vehicle. In other words, in the tire, it is planned to make an outer side of the vehicle hard, and make an inner side of the vehicle soft, and the rubbers having the different elastic modulus (storage modulus E′) are used only for achieving it. Accordingly, in an actual tire, the rubber having a high hardness is arranged outside the vehicle, and the rubber having a low hardness is arranged inside the vehicle, necessarily (refer to Tables 1 and 2 of the patent document 1).
In order to make the lane change stability, and the cornering stability and the braking performance compatible, it is important to improve the grounding property of the center rib in the condition that the severity is high, while maintaining the rigidity of the center rib, however, Patent document 1 does not teach any solving means for this at all. Even if the center rib is provided in the center portion of the tread surface and an interface of the rubbers having the different elastic modulus is formed at a center position of the center rib, in the tire mentioned above, a difference of the rubber hardness is large between the outside of the vehicle and the inside of the vehicle, and a rigidity fluctuation is generated in the center rib. Therefore, there is a risk that the lane change stability is lowered, and the incompatible phenomenon mentioned above can not be solved.